BACTERIA

Structure

Single chromosome (coiled DNA); cytoplasm: plasma membrane: cell wall. Additional structures which may be present include an external slime capsule, one or more flagella and one or more plasmids.

Brief reference should be made to plasmids as rings of DNA additional to the chromosome, that carry genes which may make bacteria resistant to one or more antibiotics, or allow them to use a wider range of food substances. For these reasons, plasmids can frequently increase the ability of bacteria to cause disease. Plasmids can replicate independently within the host bacterium. They contain DNA sequences which code for proteins that bring about conjugation, during which bacteria join temporarily by a bridge. Plasmids may move through this bridge, infecting the bacterial 'partner'. For Higher Tier students, there are links with Core and Extension material on genetic modification of organisms.
Emphasis should be placed on the cellular nature of bacteria and that, unlike viruses, the basic life processes are carried out (Core syllabus content). The size is l µm, making a convenient comparison with viruses.

Multiplication

The stages in binary fission (asexual reproduction) should be understood in terms of replication of the chromosome, cell growth, cell division, division of daughter cells. At the same time, plasmids may replicate and be present in the daughter cells.
The potential for explosively rapid reproduction as the process of binary fission gathers momentum, should be appreciated. With a generation time of 20 minutes in ideal conditions, a single bacterium might produce a population of only 8 after the first hour and 64 after two hours; after a day though, the population size would exceed 400 million million million. However, the constraints imposed by limiting factors such as nutrient supply, competition, and the production of toxic chemicals, including those which change the pH value, must be appreciated.

Students should calculate the increase in the population of a bacterium during 2 hours starting with a single cell and on the assumptions that all cells undergo binary fission a stated regular time interval and that no limiting factors operate. Students should able to evaluate data on population growth in bacteria, including plotting and interpret simple, direct (non logarithmic) graphs to include the effect of limiting factors. Students should be able to recognise and comment on the lag phase, phase of most rapid reproduction, stationary phase, and death phase of bacteria growing in culture. Students should recognise that the changing gradient of the growth curve indicates change in the rate of increase in population size.

Growth curve for a population of bacteria in a flask containing only nutrient broth

Spore formation

Students should know that some bacteria form highly resistant spores. They are 'survival' rather than reproductive or dispersal structures: their chief characteristic is a high resistance to extreme temperatures, drying up, starvation and chemicals such as disinfectants. It is difficult to destroy pathogenic bacteria which have formed spores.

This is important when considering sterilisation techniques used in such processes as food canning and preparation of surgical instruments. Many spores can survive at the temperature of boiling water for many minutes. Fortunately pathogenic spore-forming bacteria do not commonly occur in milk, hence the pasteurisation process, which does not achieve sterility, is adequate to ensure that the treated milk is safe for consumption.

Viability of spores falls off rapidly above l00 °C. In an autoclave, a time of 30 minutes is required for sterilisation at 116 °C but this falls to 2 minutes at 132 °C.